The virus that causes the mysterious and deadly new disease called Middle East respiratory syndrome may have first appeared in animals in July 2011 and then infected people multiple times. People then transmitted the virus to others, a new analysis suggests.

The illness called MERS was identified last year and has sickened 132 people, killing 58 of them, mostly in Saudi Arabia. Both bats and camels are possible carriers of the coronavirus that causes the disease, but neither is the definitive animal source of human infections.

Scientists wondered whether the virus jumped from animals to humans once or many times. To find out, Ziad Memish of the Ministry of Health in Riyadh, Saudi Arabia, and colleagues took MERS virus DNA directly from the noses and throats of 21 patients from different parts of Saudi Arabia.

Analysis of the DNA suggests the virus arose sometime between July 2007 and June 2012, with the most likely time being July 2011, Memish and his colleagues report September 20 in the Lancet. The virus may have leapt from animals to people at least seven different times, with multiple strains circulating among people at once, the researchers found.

Public health workers are still searching for infected animals that can pass the virus to humans.